1. Field of the Invention
The present invention relates to spindle lock devices for screwdrivers, and in particular to lock devices for locking a spindle of a screwdriver against a body case of the screwdriver in order to prevent rotation of the spindle.
2. Description of the Related Art
A known impact screwdriver has a spindle and an impact device that includes a hammer rotatably driven by a motor and an anvil attached to the spindle. The hammer can move toward and away from the anvil in order to intermittently apply impacts on the anvil for rotating the spindle. More specifically, when an external torque (screw tightening resistance) has applied to the anvil, the hammer moves axially away from the anvil, so that the hammer applies no impact to the anvil. Therefore, it is possible to firmly tighten screws by a predetermined tightening torque. Such a known impact screwdriver is disclosed, for example, in U.S. Pat. No. 5,016,501 and Japanese Laid-Open Utility Model Publication No. 58-160774.
However, in general, the screw tightening force is set by a compression spring that biases the hammer in the axial direction of the spindle. Therefore, it is not possible to apply a tightening force greater than a screw tightening force determined by the biasing force of the spring. Even if the entire screwdriver is rotated in the tightening direction with the motor stopped, it is not possible to further tighten the screw since the hammer will move away from the anvil and rotate relative to the anvil.
Therefore, conventionally, a manually operable screwdriver is used for further tightening a screw by a larger torque after an impact screwdriver has tightened the screw. Because a separate manually driven screwdriver is needed for further tightening the screw, the conventional design described above is inefficient and difficult to work with.
Thus, there is a need in the art for a motor driven screwdriver that can more efficiently tighten a screw after the screw has been tightened by a set tightening torque.